Compressors are used in various compression systems (e.g., refrigeration systems) to compress gas, such as freon, ammonia, natural gas, or the like, which is used to provide cooling capacity. One type of compressor is a single screw gas compressor, which is comprised of three basic components that rotate and complete the work of the compression process. These components include a single cylindrical main screw rotor with helical grooves, and two gate rotors (also known as star or star-shaped rotors), each gate rotor having a plurality of teeth. The rotational axes of the gate rotors are parallel to each other and mutually perpendicular to the axis of the main screw rotor. This type of compressor employs a housing in which the helical grooves of the main rotor mesh with the teeth of the gate rotors on opposite sides of the main rotor to define gas compression chambers. The housing is provided with two gas suction ports (one near each gate rotor) for inputting the gas and two gas discharge ports (one near each gate rotor) for entry and exit of the gas to the gas compression chambers. It is known to provide two dual slide valve assemblies on the housing (one assembly near each gate rotor) with each slide valve assembly comprising a suction valve (also referred to as a “capacity slide valve”) and a discharge slide valve (also referred to as a “volume slide valve”) for controlling an associated intake channel and an associated discharge channel, respectively. An electric motor imparts rotary motion through a driveshaft to the compressor's main rotor, which in turn rotates the two intermeshed gate rotors, compressing gas in the gas compression chambers. The compressed gas is passed to a condenser which converts the gas into a liquid. The liquid is further passed to an evaporator that converts the liquid into a gas again while providing cooling in the process.
To increase efficiency of a single screw compressor, an economizer, which is common in the industry, may be provided. The economizer function for screw compressors provides an increase in system capacity and efficiency by sub-cooling the liquid from the condenser through a heat exchanger or flash tank before it enters into the evaporator. More particularly, sub-cooling for the liquid is provided by sending high pressure liquid from the condenser into an economizer vessel through an expansion device to an intermediate pressure. The intermediate pressure in the economizer vessel is provided by an economizer port located part way in the compression cycle process of the screw compressor.
When the compressor unloads below about 60% of the full load capacity, the side/economizer port will drop in pressure level, ultimately being fully open to suction. Therefore, the liquid pressure decreases eventually down to suction pressure and no pressure difference will exist to push the liquid from the economizer vessel to the evaporator. Another side effect when the economizer port is fully opened to suction is the suction pressure will rise and the load on the compressor will need to be increased to keep the suction pressure constant.
One known method to maintain a constant economizer side port pressure is to keep the capacity slide position at 100% and run the compressor with a variable frequency drive (VFD), which can be used to unload the compressor by reducing the speed of the compressor instead of utilizing the capacity slide. Although this serves to maintain the desired pressure ratio at the economizer port, various drawbacks arise. For example, the added expense of purchasing the VFD and maintaining it is undesirable. In addition, the need for increased horsepower due to the inherent losses of the VFD can further increase cost by necessitating a larger capacity compressor. Further, the overall efficiency drops at lower speed due to the losses of the sealing effect between the internal bore and the threads of the rotor, which would allow additional gas to bypass from the high pressure side to the suction side of the compressor, and therefore increase operating costs.
Accordingly, it would be desirable to provide a method and apparatus for enhancing compressor efficiency that overcomes one or more of the aforementioned drawbacks.